Exam 3: Statistical Process Control

A pattern test can identify an out-of-control process even if all sample points are within control limits.

Variable (quantitative) control charts are used to monitor measurable characteristics of a process's outputs rather than descriptive characteristics.

Tolerances or specification limits are allowable variation prescribed in a product design.

Control charts are never implemented until special cause variation has been detected in a process.

Variable control charts are used for quantitative measures such as weight or time.

If a sample point plotted on a control chart is outside the control limits

Consider a service process that completes tax returns. An auditor has taken 20 samples each containing one completed tax return. Easy Tax is a service company that prepares tax returns. An outside auditor has examined 20 samples each containing one completed tax return. The total number of defects observed over the 20 samples is 200. The standard deviation for the control chart is

One advantage of using a pattern test is that special cause variations may be identified before any points are plotted outside the control limits.

Special cause variation can be identified using statistical process control.

Consider a production process that produces batteries. A quality engineer has taken 20 samples each containing 100 batterieslight bulbs. The total number of defective batteriesunits observed over the 20 samples is 1200. The sample standard deviation is a. 0.0183 b. 0.0224 c. 0.0265 d. 0.05210

The goal of statistical process control is to ensure that the control limits and specifications limits for a process always remain the same.

Dumplings -To-Go (DTG) provides take-out dumplings and noodle dishes to customers at its chain of drive-through restaurants. The target for a customer's waiting time is 3.0 minutes +/- 1 minute. Each month, one of the managers observes the drive-through process and collects a sample of 4 waiting times a day over a 6 day period. The data from one restaurant appears below. If DTG were to construct an X-bar chart from this data, the 3-sigma UCL would be Insert table here

If a pattern is evident in the points plotted on a control chart, the points are always considered evidence that the process is in control.

Process control charts are often used at a critical point after which it is difficult to correct or rework the process output.

Using Excel to construct control charts should be avoided because most people believe using software results in too many errors.

The range measures the variation within samples versus the variation between samples.

Marble Inc. makes countertops from a variety of high-end materials. To monitor the quality of its production processes the company randomly selects one countertop and counts the number of blemishes. The results for ten samples are shown below: Given the sample information above, the average number of defects per unit for this process would be

105. A company produces a product which is designed to weigh 10 oz., with a tolerance of + 0.5 oz. The process produces products with an average weight of 9.95 oz. and a standard deviation of 0.10 oz. According to the process capability index

An x-bar and R-chart constructed to monitor and control a process use the same raw data.

Design specification limits should always be wider than the control limits for a given process.